DC Motor Drive Tripping Often
DC Motor Drive Tripping Often
(OP)
Our client has a 440 V, 40 HP, 75A, 400 RPM base speed DC Motor driven by Eurotherm 590 + drive with 110 Amps capacity. The motor drives a rubber calendaring mill with a speed ranging from 400 RPM (field current 5.0 Amps) to 700 RPM (field current 2.4 Amps). The armature current as measured by an AC/DC clip on ammeter was found to vary from 20 Amps to 65 Amps. The current limit of the drive has been fixed at 95% of rated 75 Amps by the drive manufacturer’s rep.
This DC drive has tripped many times since its recent installation on “pulse missing”. In the field weakened range of about 650 to 690 RPM, the drive often goes to current-limit, the armature voltage is seen to vary from 200 to 570 Volts, and the field voltage from 70 to 270 volts. The drive indicates a speed error of 1.2 to 1.5%. At lower speeds, the armature and field voltages are stable.
1. What could be causing these trips on “pulse missing” ?
2. Is the current limit of 95% of rated current (as set by the drive rep) correct ? (Wouldn’t 125% be the norm for over current limits ?)
3. Why this hunting of armature and field voltages in the field weakened operation ?
4. The Eurotherm rep has advised to change the existing 4% line reactor to 2% reactor ? Is this ok ?
The same motor was previously working fine with a Siemens drive with 4% line reactor. The motor was also thoroughly checked after this tripping problem and found to be alright in all aspects.





RE: DC Motor Drive Tripping Often
1. What could be causing these trips on “pulse missing” ?
///Improper timing. Relaxation oscillator, if present, is less accurate, i.e. fast or slow.\\\
2. Is the current limit of 95% of rated current (as set by the drive rep) correct ? (Wouldn’t 125% be the norm for over current limits ?)
///This may be set for sensitive protection of the motor, if the motor shaft is not fully loaded. However, the drive protection should be according to the drive spec sheet or manual.\\\
3. Why this hunting of armature and field voltages in the field weakened operation ?
///Apparently, the motor shaft load is bigger than the drive delivered energy.\\\
4. The Eurotherm rep has advised to change the existing 4% line reactor to 2% reactor ? Is this ok ?
///This may reduce the voltage drop across the reactor; however, the harmonics will increase on the drive input, if the reactor is on the drive input. If it is on the drive output, then the voltage drop will be reduced; however, the harmonic content will increase.\\\
RE: DC Motor Drive Tripping Often
If can look at the DC waveform, you will see a periodic missing peak when one phase of the AC side is not carrying load.
What you may also find, on the AC side, is a current draw on only side side, i.e., negative or positive. This would confirm a rectifier device fault.
I have encountered this type of fault on several occasions, so perhaps you have something like it.
RE: DC Motor Drive Tripping Often
RE: DC Motor Drive Tripping Often
RE: DC Motor Drive Tripping Often
RE: DC Motor Drive Tripping Often
I am certain that the problem is at component level on the drive my next move is to get a Eurotherm guy in to strip it down and start swopping out boards.
RE: DC Motor Drive Tripping Often
My client just informed me that Euortherm rep disabled the "Pulse Missing" trip. Apparently, he required some password from the HQ in London (?) to do this. Still, he could not explain why the trip occurs in the first place. May be your Eurotherm guy can do better. If so, pls share it in the forum.
RE: DC Motor Drive Tripping Often
I believe the Eurotherm HQ is in Littlehampton on the south coast of England.
RE: DC Motor Drive Tripping Often
RE: DC Motor Drive Tripping Often
Do you know what pulse missing means?
Did you obtain current or waveforms to ensure that rectification is occuring properly. You can also obtain vibration readings. Improper rectification can result in high vibration at 10,800 or more likely 21,600 CPM.
I just had a case where a DC motor was reportedly producing a large magnetic field that interefered with instrumentation. My analysis revealed an increase in 21,6000 CPM vibration from .05"/sec to .41"/sec. The DC current waveform revealed extreme peaks of extremely short duration, rather than half cycle duration for each ripple. (Too bad I can't attach the waveforms here, they are very interesting.) In addition, one ripple was actually negative going, suggesting the motor was enduring reverse torque. They had to slow the motor down to keep it on line.
They are now having the drive manufacturers look for a timing or other fault in the drive. The SCR's or whatever is being used in this drive for rectification is clearly not staying open long enough and in the proper sequence.
The bottom line here is that you can learn a lot about the drive operation from both the vibration signatures and the current signature.
RE: DC Motor Drive Tripping Often
The motor vibration levels at various speeds and loads do not exceed 0.2 mm/sec. By hand, you can feel the motor running very smoothly. Maximum motor temp is only 58 deg C.
Have you had any such expereinces with the drive mentioned ?
RE: DC Motor Drive Tripping Often
That vibration level is so low, you have got to wonder if the machine is running!
In the past, I had a drive that ran heating elements at a computer chip plant. The drive continuously went out on overload. Examination of the input and output waveforms revealed a "pulse missing" in the sense that AC on one phase was not drawn on half a cycle. The resulting waveforms had a "pulse missing". The drive tried to make up the load by increasing the load on the other two phases. This increased the loads sufficiently to trip the drive.
If you go to page 7-12 in the Eurotherm manual, http://www.eurothermdrives.com/doc/HA470388.pdf
it describes a missing pulse. This condition means that one of the six pulses is missing from the the armature waveform and you probably have a defective SCR or SCR control. This is the condition I was referring to on my first reply to this thread.
Once again, check the DC current waveform using an o-scope.
RE: DC Motor Drive Tripping Often
Another commment. Since the vibration level is so low, you may not actually have a waveform fault. The page cited above also lists control related faults that may occur even if the waveform is properly formed. It seems that these control related functions might be the place to look. But, the easy starting point is checking the waveform.
RE: DC Motor Drive Tripping Often
RE: DC Motor Drive Tripping Often
RE: DC Motor Drive Tripping Often
all modes of operation, not just in field weakening.
The pulse missing fault is most likely caused by some feed-
back element.This raises the question of how the drive gets
speed feedback;tachometer,encoder, or "armature" feedback.
Also could you describe what type of current feed back your system uses.Also I assume you are using a speed control mode of operation as opposed to constant current.
The fault could also be in your "zero crossing" circuit.
This citcuit lets the gate curcuit know when the observed phase is rising from zero volts so the gate can be fired at the calculated angle of the cycle. You could check this with
a dual trace scope connected at the zero crossing test point and the input of the phase to the drive for problems with sync pulses on the z.c. side not matching with the phase zero crossing.
You current limit is set to high at 95% rating of the drive.
You "current limit" should never be set above moter rated full load. Forget about inrush as if you were sizing a curcuit breaker.
I think the hunting of voltages in your armmature and field circuit is normal. Your drive is just tring to keep the speed of you motor constant under demanding conditions.
I dont think the reactor change will help this problem.
Overall though this sounds like poor drive tuning.
RE: DC Motor Drive Tripping Often
RE: DC Motor Drive Tripping Often
RE: DC Motor Drive Tripping Often
electromike,
The current limit is set at 95% of motor current of 75 Amps. The drive is rated for 110 Amps. I think the current limit should be set to atleast 110% of motor current to avoid frequenct "Current Limit" operations. Do you agree ?
RE: DC Motor Drive Tripping Often
RE: DC Motor Drive Tripping Often
RE: DC Motor Drive Tripping Often
To anyone else with a missing pulse, if it is possible, can you monitor the current feedback waveform? Preferably with a storage scope. I would like to know exactly how many pulses are missing, if in fact they are missing.
Also, make sure all of the gate leads which go to the Scr's are secure. I have seen in one case where this was a problem on a drive mounted in a high vibration area.
RE: DC Motor Drive Tripping Often
Just in case you are not aware of this, if one Scr is open or not firing, for example, you will not be missing 1 pulse on a 3phase, 6 pulse system. You will, in fact, be missing two consecutive pulses.
RE: DC Motor Drive Tripping Often
Your operating current limit should be a percentage of the maximum current of the drive. Which in this case is 110A. So at 95% of 110A your drive will allow your motor to run at 104.5A. Most drives have a parameter that limits the time a drive will run in limit. The term current limit also seems to be getting confused with what should be the current trip of the drive. Some drives allow you some range in setting current trip. Keep in mind when you hit current trip your drive shuts down instantly.(At least your drive stops instantly, your conducting scrs will try to carry the high current until the holding current drops below the given value for the scrs. Not good.) The reason for the instant shut down is to protect your scrs,,semi-fuses, and load. This is the way the systems I know operate and I suspect this one is no different. You can exceed the full load current for a very short time without heat and commutation problems in D.C. motors. I would check armature current when the drive is in limit to investigate what the current is realy doing.
RE: DC Motor Drive Tripping Often
Also, the ambient temperature is not mentioned in the original posting. The high ambient temperature can cause electronic circuits to have changed operating points and quiescent points, which can have detrimental impact on the gate triggered switching devices.
RE: DC Motor Drive Tripping Often
Thx again for your time and views.